Fuel cell having sintered porous electrode consisting of electrically conductive material and of boron



1968 l. LINDHOLM ETAL ,41 ,438

FUEL CELL HAVING SINTE-RED POROUS ELECTRODE CONSISTING OF ELECTRICALLYCONDUCTIVE MATERIAL AND OF BORON Original Filed Sept. 21, 1964 INVENTOR.

United States Patent 3,414,438 FUEL CELL HAVING SINTERED POROUS ELEC-TRODE CONSISTING OF ELECTRICALLY CON- DUCTIVE MATERIAL AND OF BORONIngemar Lindholm, Bo Mueller, and Olle Mjorne, Vasteras, Sweden,assignors to Allmiinna Svenska Elektriska Aktiebolaget, Vasteras,Sweden, a corporation of Sweden Continuation of application Ser. No.397,696, Sept. 21, 1964. This application Jan. 30, 1967, Ser. No.612,730 Claims priority, application Sweden, Sept. 27, 1963, 10,557/ 636 Claims. (Cl. 13686) ABSTRACT OF THE DISCLOSURE A fuel cell with atleast one porous electrode consisting essentially of a body materialproduced by sintering a mixture of particles of an electricallyconductive material and of boron, and by possibly dissolving out theboron, at least partially of the body material.

This application is a continuation of application Ser. No. 397,696,filed Sept. 21, 1964, now abandoned.

Electrical energy can be generated inter alia by means of reactionbetween a continually supplied combustible substance such as for examplehydrogen and a continually supplied oxidising substance, such as forexample oxygen, air or a halogen in a so-called fuel cell, which in itsmost simple form may consist of a suitable fluid electrolyte and twoporous electrodes immersed in this, one of which is arranged between theelectrolyte and the combustible substance and the other between theelectrolyte and the oxidising substance.

The electrode reactions in the fuel cells occur at the points of contactbetween electrolyte, combustible and oxidising substances respectivelyand the electrode. These points of contact are situated at the points inthe electrodes where electrolyte and combustible and oxidisingsubstances respectively are adjacent to each other. The points in theelectrodes which are active when a reaction occurs are thereforesituated in the pore surfaces.

A porous electrode in a fuel cell need not form a separating wallbetween a gas chamber containing a gaseous substance, such as a fuel,and an electrolyte chamber containing a fluid electrolyte. Thecombustible substance may namely be dispersed or dissolved in theelectrolyte, as is the case with cells for fluid fuel and then theelectrolyte is present with its fuel on both sides of the electrode aswell as in its pores. Certain oxidants, for example hydrogen peroxide,may also be dissolved in the electrolyte and then the conditions on theoxidant side are analogous to those described for the electrode on thefuel side in fuel cells with fluid fuel.

In fuel cells it is known to use electrodes which in the main are formedof nickel and in which the areas nearest the pore walls contain nickelin activated form. In the parts of the electrodes situated outside thementioned areas, nickel is present in inactive form and its function isthen to act as a carrier material for the active areas situated nearestthe pore walls. Such an electrode may according to a known method beproduced by using nickel powder and a powder consisting of analuminium-nickel alloy (Raney alloy). A mixture of the powder is thenpressed together to an electrode, which is then sintered. After thesintering the aluminium is dissolved out of the aluminium-nickel alloy(Raney alloy) in the sintered product with alkali, mic-ropores thusbeing formed. The area around the pores becomes active due to the largepore surfaces and the dis- 3,414,438 Patented Dec. 3, 1968 turbedcrystal lattice, which the remaining material has after the dissolutionof the aluminium. As previously indicated, after the sintering the purenickel powder acts as a carrying body for the elect-rode, while thematerial of the alloy remaining after the dissolution with alkali formsthe active area which surrounds the pores. As a substitute for the purenickel powder for the known electrodes, carbon, iron and cobalt powderhave been proposed and as a substitute for the said alloy, other alloys,in which nickel can be replaced by cobalt or iron, and aluminium bysilicon, magnesium or zinc. Among other things the electrodes can beused as fuel electrodes for hydrogen.

In the US. patent application Ser. No. 191,120 there has moreover beenproposed a method of manufacturing porous electrodes by sinteringtogether a mixture of particles of nickel and of aluminium andsubsequently at least partial dissolving the aluminium out of theproduct sintered together. The nickel powder can also be replaced byiron, cobalt, molybdenum, tungsten or silver powder and the aluminiumpowder by magnesium, zinc or silicon powder in this product.

It has now been proved possible to manufacture an electrode materialwhich has essential advantages over those earlier known or proposed. Theelectrode material manufactured according to the present invention isthus resistant against air and can, contrary to the electrode materialsdescribed above, be kept in open air without difficulty. Anotherimportant advantage of the electrode material is that it withstands highoperating temperatures which, when the electrode material is used inhydrogen oxygen cells, means that the removal of the water formed by thereactions is facilitated. Another advantage of great significance isthat only a very small amount of an activating material is required forthe manufacture of the electrode material. An important advantage isfurther that the electrode material can be used directly after thesintering and need not be subjected to any subsequent treatment, such asa dissolution process with alkali. That brings an importantsimplification of the production, since the dissolution processconstitutes in itself a very troublesome procedure because of the largequantity of alkali which must be used, the strong evolution of hydrogenwhich occurs and the pyrophoric nature of the material resulting fromthe treatment.

The invention relates to a method of manufacturing a porous electrodematerial, preferably for use in fuel cells, in which a powder mixturecomprising particles of an electrically conducting material, for examplenickel, and particles of a material activating the conducting materialare sintered together to a coherent product. The method is characterizedin that boron is used as activating material and in that the boron maybe at least partially dissolved out of the sintered product.

The electrically conducting material may preferably comprise one of thesubstances iron, nickel, cobalt, silver, tungsten or molybdenum ormixtures of these substances.

In order to improve the activity of the manufactured electrode materiala small amount of a compound of one or several of the metals chromium,molybdenum, tungsten, titanium, aluminium, thorium, cobalt or platinumcan be supplied to the powder mixture which is to be subjected tosintering, The compound of the metal in question will be of such a typethat it gives oxide formation during sintering. In this way smallamounts, less than 2 percent by weight, of these metals are incorporatedin the sintered electrode material. An electrode material withespecially high activity is obtained if the sintering is performed in ahydrogen atmosphere. A possible explanation of this may be that boronhydride may then be formed as an intermediary product which can reactwith the conducting material at all its occurring surfaces.

The powder mixture which is to be sintered consists suitably for themain part of particles of the electrically conducting metal. The boronpercentage in the powder mixture lies suitably at 01-10 percent byWeight, preferably at 0.5 to 5 percent by weight, based on the totalweight of the powder mixture.

The particle size of the particle materials may be varied within widelimits depending on the fuel and electrode type in question. The size ofthe pores in a manufactured electrode is determined to a great extent bythe size of the particles used. In most cases it is desirable that allthe pores are the same size, which is attained through the use of welldemarcated powder fractions. In many cases it is suitable to use powderfractions with an average particle size of 0.5-50 for the boron andpowder fractions with an average particle size of 1-50,u. for theconducting material. With the use of nickel as conducting material ithas been found especially advantageous to use powder fractions with anaverage particle size less than for the boron and powder fractions withan average particle size less than 25,41. for the nickel. in order toincrease the porosity of the electrode there may be added to the mixtureof the particles up to 25 percent by weight of an expanding agent, forexample, ammonium carbonate or ammonium bicarbonate, capable of beingdissociated into gaseous products during the sintering process.

The sintering of the powder mixture to an electrode material mayadvantageously in many cases be done at a temperature of around 500-1500C. depending on the kind of the conducting material.

If a porous electrode material produced by sintering together aconducting material of the above mentioned type and boron is placed inan alkaline liquid, an at least partial dissolution of the boron out ofthe electrode material takes place slowly. In certain cases such adissolution can alsotake place in acid or neutral liquids. It has beenfound that after such a dissolution the electrode material is at leastas active as before the dissolution. Such a dissolution of the boron isautomatically obtained if an electrode material with boron on the poresurfaces is used in a fuel cell with an alkaline liquid, for example,potassium hydroxide, as the electrolyte or in another arrangement withalkaline electrolyte, in which electrode reactions with hydrogen as aparticipant part are utilized for the generation of electrical energy.Of course, if so desired, the said dissolution can be carried out beforethe electrode material is arranged in the fuel cell or in the saidarrangement.

The invention may be further explained in connection with thedescription of a number of embodiments:

In this explanation reference is made to FIG. 1 and FIG. 2 showing anenlargement of a small part of an electrode material according to theinvention before and after the sintering operation respectively.

In FIG. 1, 10 designates particles of an electrically conductingmaterial, e.g., nickel and 11 particles of boron. Between the particleswhich are compressed there are open spaces forming together pores 12.After the sintering the particles 10 of electrically conducting materialare provided with surface layers 13 containing boron. The boron mayeventually be dissolved out by a solvent such as potassium hydroxide. Insuch a case active surface layers remain on the electrically conductingmaterial. 14 designates an electrolyte penetrating the electrodematerial from one side and 15 a combustible gas penetrating theelectrode material from the other side. The electrolyte and the gas forma boundary 16 in the pore.

an average particle size of 2 am. is mixed with 99 percent by weight ofcarbonyl nickel powder with an average particle size of 5 ,um. Themixture is pressed to electrodes with a pressure of 1000 kiloponds/cm.and is sintered in a hydrogen atmosphere at 600 C. for 30 minutes.

Example 2 0.5 percent by weight of amorphous boron powder with anaverage particle size of 2 ,um. is mixed with 98.5 percent by weight ofcarbonyl nickel powder with an average particle size of 5 1.111. and 1percent by weight of chromium hydroxide powder with an average particlesize of 5 ,um. The mixture is pressed to electrodes with a .pressure of1000 kiloponds/cm. and is sintered in a hydrogen atmosphere at 700 C.for 30 minutes.

Example 3 2 percent by weight of amorphous boron powder with an averageparticle size of 2 ,urn. is mixed with 88 percent by weight of carbonylnickel powder with an average particle size of 5 pm. and 10 percent byweight of ammonium bicarbonate powder with an average particle size of40 am. The mixture is pressed to electrodes with a pressure of 2000kilo-ponds/cm. and is sintered in a hydrogen atmosphere at 800 C. for 30minutes.

Example 4 4 percent by weight of boron powder with an average particlesize of 5 am. is mixed with 96 percent by weight of nickel powder withan average particle size of 10 m. The mixture is pressed to electrodeswith a pressure of 1000 kiloponds/cm. and is sintered in vacuum at 900C. for 30 minutes.

EXAMPLE 5 1 percent by weight of amorphous boron powder with an averageparticle size of 2 ,um. is mixed with 99 percent by weight of molybdenumpowder with an average particle size of 30 m. The mixture is pressed toelectrodes with a pressure of 1000 kiloponds/cm. and is sintered invacuum at 1000 C. for 60 minutes.

EXAMPLE 6 2 percent by weight of amorphous boron powder with an averageparticle size of 2 ,um. is mixed with 98 percent by weight of carbonyliron powder with an average particle size of about 20 ,um. The mixtureis pressed to electrodes with a pressure of 2000 kiloponds/cm. and issintered in a hydrogen atmosphere at 800 C. for 30 minutes.

For all the electrode materials produced according to the examples it isfound that after they have been used as fuel electrodes in a fuel cellwith potassium hydroxide, for example a 30 percent potassium hydroxidewater solution as an electrolyte and thereby at least a partialdissolution of the boron has taken place, they show an activity which isat least as great as the original one. Not even after use for a longtime, when nearly all boron may be dissolved, do the electrodes show adecreased activity.

The electrodes described could be used in fuel cells with differentelectrolytes, such as for example potassium or sodium hydroxidesolutions. The electrode material may not only be formed into plates butalso amongst other things as pellets, grains or the like. Such electrodemate rial in pellet or grain form is used, inter alia in fuel cells withfluid fuel, for example alcohol or hydrazin dissolved in theelectrolyte, in which case for example it may be arranged in a containermanufactured of a net or perforated sheet metal. The electrode materialmay also be used in fuel cells of the type Where the electrolyteconsists of an ion exchange membrane of organic or inorganic yp Theelectrodes described may with advantage be used not only in fuel cellsbut also in other types of arrangements where electrode reactions withhydrogen as a participant part are used for generation of electricalenergy, for example in accumulators.

We claim:

1. A fuel cell having electrodes in contact with an electrolyte, atleast one of said electrodes being porous, and permeable to gas, andconsisting essentially of a sintered mixture of particles of anelectrically conductive material and of boron; the percentage of boronbeing 0.1-10 percent by weight of the sintered mixture, saidelectrically conducting material being at least one substance selectedfrom the group consisting of iron, nickel, cobalt, tungsten andmolybdenum.

2. A fuel cell as claimed in claim 1, in which the boron is at leastpartially dissolved out of the sintered mixture.

3. A fuel cell as claimed in claim 2, in which the electricallyconducting material consists essentially of nickel.

4. A fuel cell as claimed in claim 2, in which the electricallyconducting material consists essentially of cobalt.

5. A fuel cell as claimed in claim 1, in which the electricallyconducting material consists essentially of nickel.

6. A fuel cell as claimed in claim 1, in which the electricallyconducting material consists essentially of cobalt.

References Cited UNITED STATES PATENTS 1,030,666 6/1912 Kuzel 75-2071,648,679 11/1927 Fonda 252518 X 2,059,041 10/1936 Schroter et al. 75202X 2,073,826 3/1937 Balke 75--202 2,725,287 11/1955 Cronin 752002,769,114 10/1956 Williams 252-512 X 2,860,175 11/1958 Justi 75222 X2,936,250 5/1960 Glaser 10666 X 3,151,386 1-0/1964 Ingersoll 75-202 X3,183,123 5/1965 Haworth 136-86 3,183,124 5/1965 Jasinski 136-86 X3,202,862 8/1965 Paley 252512 X WINSTON A. DOUGLAS, Primary Examiner.

A. SKAPARS, Assistant Examiner.

